Jet ignition in small automotive engines: development, design and testing of innovative passive combustion prechambers

During the research activity, the possible advantages of using an innovative passive prechamber were studied. First the passive prechamber applying it to a two stroke LPDI (Low Pressure Direct Injection) engine with developed by the REASE research group in collaboration with Piaggio S.p.A. and afterwards on a four stroke PFI (Port Fuel Injection) engine in partnership with Betamotor S.p.A. and HPE COXA S.p.A. For what concern the two-stroke LPDI engine the experimental tests showed that the use of a combustion prechamber offers advantages in some areas of engine operation over the standard version. More specifically, the advantages occur when the engine load is high enough not to generate misfire phenomena, allowing a higher speed and completeness. Furthermore, at high engine loads the use of JI technologies allow to reduce the engine cycle-to-cycle variation, which results particularly important in two-stroke engine. Different is the case at low operating loads where incomplete scavenging of the prechamber volume does not allow for stable engine operation. In this engine, a sensitivity analysis was also carried out varying the main prechamber geometries, i.e., internal prechamber volume, prechamber sinking and orifices diameter and number. The results further show that, among the various types of prechambers tested, in order to achieve faster combustion, it is necessary to maintain a high internal prechamber volume. As the engine speed increases, prechambers with larger equivalent bore diameters are preferable. Regarding the four-stroke engine, the aim of the study was to investigate the possibility of increasing the performance by exploiting the potential of prechamber rapid combustion to extend the engine's operating limits in terms of knock. Test results showed a significant increase in burning rate at high engine operating loads. This allows a reduction of cycle-to-cycle variation, a rise of the knock limit and the capacity of the engine to operate with leaner air-to-fuel mixtures. Direct consequence of this is expected to be an increase of the engine thermodynamic efficiency but actually a limited increase in indicated mean effective pressure was observed. So, further investigations were necessary to fully understand this unforeseen behaviour. Since from the literature it is known that the use of the prechamber strongly increases the turbulence, the research activity was focused on an experimental investigations of the combustion chamber thermal exchange. More specifically, the sensible heat losses, the chemical exhaust losses and the liquid cooling system losses were measured under several engine operating conditions. This investigation showed that the use of the prechamber causes an increase in the heat released to the cooling system which leads to a deterioration of the expansion phase compared to the engine in standard configuration. As far as engine operation at medium and low loads is concerned, exactly as observed in the work done on two-stroke engines, there main problem is related to the scavenging process of the prechamber resulting in an unacceptable cycle-to-cycle variation of the engine. To deal with the problems that emerged during the experimental campaign, further investigation was required, which led to the numerical development of two solutions. With regard to the problem of increased heat exchange, it has been hypothesised to make the combustion chamber more adiabatic by applying a ceramic coating treatment to the head, valves and piston crown. This is made possible by the fact that, as seen experimentally, the use of a prechamber allows the knock limit to be raised. On the other hand, with regard to the problems encountered at low load, a new innovative prechamber geometry has been proposed and numerically investigated which has a calibrated scavenging duct in communication with the external environment. This orifice is positioned in such a way as to allow the prechamber to be scavenged even at partial loads without, however, generating significant energy losses during the combustion phase.